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F2.4 · Investigate properties of magnetic fields

Learn to investigate properties of magnetic fields through clear examples and targeted practice.

Ontario Grade 11 Physics

Electricity and Magnetism

SPH3U study topic F2.4

A magnet can affect another magnet without touching it. To describe this influence, physicists use the idea of a magnetic field: the region around a magnet or electric current where magnetic effects can be detected. This lesson focuses on investigating field patterns and deciding what those patterns show. Arrows and field-line spacing are ways to represent a field. They are not physical objects in space. Before interpreting a diagram, identify what is being studied and choose a consistent viewing direction.

What you will learn

1. Prerequisite bridge: directions, vectors, and evidence

A scalar has size but no direction. Temperature is a scalar. A vector has both size and direction. Magnetic field is a vector quantity: it has a direction and a strength. At a point, the field direction is the direction in which the north-seeking end of a small compass points.
A compass needle is a small magnet. It turns to align with the field where it is placed. A compass therefore gives evidence about local field direction. A compass by itself does not give a numerical value for field strength.
A field diagram is a model used to communicate a field pattern. Arrows show direction. Lines drawn closer together represent a stronger field than lines farther apart. This spacing is a qualitative comparison, not a measurement. Field lines are not tracks followed by tiny particles.
For an investigation, define the system: the magnet or wire being studied and the surrounding region where its magnetic effect is explored. Choose a fixed viewing direction, such as looking down at a sheet of paper. Mark the positions of the magnet, wire, and compass. These choices help make observations and diagrams clear.

2. Investigating a permanent magnet

A permanent magnet has two poles, called north and south. Like poles repel, and unlike poles attract. By convention, the magnetic field outside a bar magnet is represented as leaving the north pole and entering the south pole. A compass placed at different positions can show the local direction of that pattern.
To map a field with a compass, place the bar magnet beside or beneath a sheet of paper and trace its outline. Put a compass at one position near the magnet. Mark the direction of the compass's north-seeking end. Move the compass so its tail begins at the mark, then mark the new direction. Repeat to build a chain of direction marks. Draw a smooth line through the marks and add arrows. Start at several positions to show more of the pattern.
Iron filings offer another way to reveal the overall pattern. A proposed procedure is to place the magnet beneath a sheet of paper, sprinkle filings lightly on top, and tap the sheet gently. The filings tend to line up with the field pattern. This method can show the shape, while a compass more clearly shows direction.
A proposed procedure is not evidence that the investigation has been done. Evidence consists of observations actually made and recorded. Keep the magnet in the same position while mapping. Keep other magnets and magnetic objects away because they may affect the pattern. Record the method, positions, and observations.

3. Investigating the field around a current-carrying wire

An electric current is a flow of electric charge. A wire carrying current produces a magnetic field in the space around it. A nearby compass can turn in response to that field. In a simple investigation, keep magnets and other current-carrying wires away so they do not confuse the observation.
Around a straight wire, the field pattern circles the wire. The right-hand grip rule connects conventional current direction to field direction. Conventional current is the direction positive charge would move in a circuit. Point your right thumb along that direction; your curled fingers show the direction of the magnetic field around the wire. This rule uses conventional current, not electron motion.
For a clear direction description, imagine a wire passing through a flat page. Looking at the page, current directed toward you comes out of the page. With your right thumb pointing toward you, your curled fingers show the field circling counterclockwise. At the point to the right of the wire, the field points upward on the page. If current is directed away from you, the field direction reverses.
A possible investigation is to pass a straight wire through a hole in a flat card and connect it to a low-voltage power supply. Place a compass at marked positions around the wire and briefly switch on the current. Record the needle direction at each position. Reverse the current and repeat without moving the wire or compass. The prediction is that the field direction reverses. This is a prediction until observations are made and recorded.
Use the power supply only as directed by a teacher. A current can heat a wire, so do not leave the circuit connected longer than needed. Change one feature at a time to make the comparison useful.

4. Reading diagrams and making careful comparisons

A field diagram represents two properties. An arrow at a point shows field direction. The relative spacing of nearby lines suggests field strength. More crowded lines represent a stronger field than more widely spaced lines. Do not read an exact field strength from a sketch unless it is supported by a suitable measurement and scale.
The SI unit for magnetic field strength is the tesla, written as T\mathrm{T}. The investigations in this lesson focus on qualitative patterns, so they do not require a numerical field-strength measurement. If an instrument provides a numerical reading, report the value with its unit and state where and how it was measured.
When comparing diagrams, first check that they use the same conventions. For example, arrows should show field direction in both diagrams, and line spacing should represent strength in the same way. Then compare pattern shape, direction, and relative spacing. State whether each conclusion comes from an observation, such as a compass turning, or from an interpretation of a model, such as closer lines representing a stronger field.
A useful investigation question is: How does reversing current affect the field around a straight wire? Keep the wire and compass positions fixed. Record the current direction and compass direction for each position before and after reversing the current. The recorded needle directions are the evidence. A field diagram made from them is a model of the observations.

Worked example

Example 1: Interpreting a bar-magnet field

A field diagram shows arrows outside a bar magnet pointing away from the left pole and toward the right pole. The lines are closer together near both ends. Identify the poles and compare the represented field strength.
  1. Set the system and convention
    The system is the bar magnet and the space around it. Use the convention that arrows show the field direction outside the magnet.
  2. Identify the poles
    Outside a bar magnet, the field is represented as leaving the north pole and entering the south pole. The left pole is therefore north, and the right pole is south.
  3. Compare the regions
    Closer line spacing near the ends represents a stronger field there than in a region where the lines are farther apart. This is a qualitative comparison, not a numerical measurement.
Answer: The left pole is north and the right pole is south. The diagram represents a stronger field near the ends than in less crowded regions.
Check: The arrows point from north to south outside the magnet. The strength comparison follows the stated line-spacing convention and does not claim a numerical measurement.

Worked example

Example 2: Using a compass near a wire

A straight wire passes through a page. Looking at the page, conventional current comes out of the page toward you. A compass is to the right of the wire. Which way should its north-seeking end point, according to the right-hand grip rule?
  1. Define the view and system
    The system is the current-carrying wire and nearby compass. The viewing direction is toward the page. Current coming out of the page is directed toward the viewer.
  2. Apply the rule
    Point your right thumb toward you, along the conventional current. Your curled fingers show a counterclockwise field when viewed facing the page. At the wire's right side, the field points upward on the page. The compass north-seeking end aligns with this local field direction.
Answer: The compass north-seeking end points upward on the page at the wire's right side.
Check: The direction follows the right-hand grip rule for current coming toward the viewer. Reversing the current would reverse the field and the predicted compass direction.

Worked example

Example 3: Planning a fair comparison

A student wants to test whether reversing current reverses the magnetic field around a straight wire. State what to hold constant, what to change, and what to record.
  1. Define the system and direction
    The system is the wire, power supply, and nearby compass. Choose one viewing direction and mark the wire and compass positions before taking observations.
  2. Choose the changed feature
    Change only the current direction between trials. Keep the wire and compass positions fixed so that a change in setup does not explain a change in needle direction.
  3. Record evidence
    For each current direction, record the current direction and compass direction at every marked position. Compare the observations. Reversal is supported if the compass directions reverse at corresponding positions.
Answer: Keep the wire and compass positions fixed, reverse the current, and record current direction and compass direction for both trials.
Check: The comparison changes one feature at a time. The conclusion must be based on recorded observations, not on the prediction alone.

Common mistakes and how to avoid them

Treating field lines as physical strings or paths that objects must follow.
Correction: Field lines are a drawing convention. They represent field direction and relative strength.
Drawing field arrows from south to north outside a bar magnet.
Correction: Use the convention that outside the magnet, arrows leave north and enter south.
Saying that a compass measures field strength.
Correction: A compass indicates direction. A numerical field-strength value requires a suitable measuring instrument.
Calling a prediction experimental evidence.
Correction: A prediction describes what is expected. Evidence is what an investigation actually observes or measures.
Using electron motion instead of conventional current in the right-hand grip rule.
Correction: Point your right thumb along conventional current. Reversing that direction reverses the represented field direction.

Lesson summary

Check your understanding

Question 1

A compass is moved around a bar magnet. What does the north-seeking end show at each position?
  1. The local magnetic field direction
  2. The numerical magnetic field strength in tesla
  3. The magnet's temperature
  4. The direction of electron flow in the compass
Show answer and explanation
The local magnetic field direction
The compass aligns with the local field direction. A compass alone does not give a numerical field-strength reading.

Question 2

For a straight wire, what happens to the magnetic field direction when the conventional current is reversed?
  1. It reverses around the wire.
  2. It stays unchanged.
  3. It changes into a field that points only along the wire.
  4. It disappears permanently.
Show answer and explanation
It reverses around the wire.
The right-hand grip rule links field direction to conventional current. Reversing the current reverses the field direction.

Question 3

What does closer spacing of field lines represent in a field diagram?
  1. A stronger field relative to a region with wider spacing
  2. A weaker field relative to a region with wider spacing
  3. A measured value in tesla without an instrument
  4. A place where the field direction is undefined
Show answer and explanation
A stronger field relative to a region with wider spacing
Line spacing represents relative strength. It does not by itself provide a numerical measurement.

Key terms

Magnetic field
The region around a magnet or electric current where magnetic effects can be detected; it has direction and strength.
Vector
A quantity with both size and direction.
Scalar
A quantity with size but no direction.
Conventional current
The chosen direction for current in a circuit, defined as the direction positive charge would move.
Field diagram
A drawing that represents field direction with arrows and relative strength with line spacing.
Tesla
The SI unit of magnetic field strength, written as T\mathrm{T}.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Physics (SPH3U), expectation F2.4. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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